Sleeve type connecting rod bearings have long been used in internal combustion engines. Among other things their purpose is to transfer the power generated in the combustion chamber to the crankshaft from whence it is transferred directly to the engine flywheel and finally to some sort of drive train assembly. The purpose of the connecting rod bearing is to reduce friction between the connecting rod and the constantly cycling crankshaft on which it is journaled. Generally, the bearing surface material is an alloy having good lubricity, and good load carrying and wear characteristics, such as aluminum-tin, aluminum-lead, copper-tin and the like. Further, it is customary for most vehicular applications that the bearing surface material be metallurgically bonded to a thin steel backing strip. In any case it is well known these bearings must be constantly lubricated with oil and that to do so a clearance space must be provided between the crankshaft and the bearing surface such that the oil, which is generally introduced into the clearance space from the crankshaft, will completely fill the clearance space and constantly lubricate the bearing surface. It is also well known that if, because of journal loading, the bearing is eccentrically positioned on the crankshaft journal, the rotational movement between these two members will tend to draw the oil between the loaded faces and thereby create an oil film or wedge. This hydrodynamic wedge effect of the oil within the clearance space floats the bearing on the journal which is, of course, a desirable result. Such an oil film increases in thickness and effectiveness as the clearance space decreases. By way of example, in a typical vehicular application wherein there is used a conventional connecting rod bearing having a bearing surface concentric with the outer surface of the bearing sleeve and the center of the journal member the maximum oil film thickness is measured in terms of millionths of an inch. For this same application there is usually provided a clearance space between the crankshaft diameter and the bearing crown diameter ranging from one to four thousandths of an inch. Thus, while there is a distinct relationship between clearance space and oil film thickness, the two are not the same. The "bearing crown diameter" is the bearing inner diameter or bearing surface diameter measured along a diametric axis beginning at the point the greatest load will be exerted on the bearing.
Others have recognized the advantage of designing a connecting rod bearing such that the inner bearing surface is eccentric to the outer bearing surface and crankshaft journal in such a manner that, within the circumferential region of the bearing surface which is subjected to the greatest load, the clearance space is less than that of the above-described conventional bearing. Consequently the oil film thickness can be and is increased to its maximum load carrying capability. For typical vehicular applications as above-described an oil film thickness ranging from 25 to 50 millionths of an inch can be achieved. Such a design also offers the advantage that as a consequence of the clearance space being reduced in the region of the greatest load, the oil film pressure and thus the fatigue on the bearing material is also reduced.
One example of eccentrically designed engine connecting rod bearings is disclosed in U.S. Pat. No. 4,073,550 to Yahraus in which a thick walled half shell is provided in the high load area of the bearing and a thin walled bearing is provided in the low load portion of the bearing. In this construction an edge or lip of the thick walled bearing extends into the oil clearance area. Unfortunately, this noncontinuous surface tends to shear the oil film in this area and thereby decrease the effectiveness of the oil wedge.
U.S. Pat. No. 4,307,921 to Roberts and assigned to the assignee of this invention discloses another design intended to tailor the clearance between the journal member and bearing surface to the loading and cooling requirements of an engine bearing. The Roberts patent discloses a sleeve bearing including a bearing surface having a first portion which is concentric with the journal member and has a constant radius closely approximating the radius of the journaled member. A second portion of the bearing surface extends circumferentially outside the constant radius of the first portion to increase clearance between the bearing surface and journal member at a low load portion of the bearing. This construction quite effectively provides adequate clearance for oil in the area defined by the second portion while minimizing oil clearance and pressure about the first portion. However, the manufacture of such a bearing is complex in that two radii must be generated on each bearing half. This may be done by means of a special broach or by double boring or double broaching the bearing surface, and each such means is considered less desireable than the methods to which the present invention lends itself. Another possible disadvantage of this design is that the first and second portions of the bearing surface meet at spaced points about the circumference of the bearing and define a noncontinuous surface at that point.